1. Technical Field
This application pertains to filters and filtration processes and systems generally and, more particularly, to the enablement of the use of deep filter media used in ionizing electrically enhanced filtration processes and filters while functioning as high performance devices with ultra-low pressure drop, to filtration systems and to processes or constructing filters and filtration systems.
2. Related Art
Jaisinghani, A Safe Ionizing Field Electronically Enhanced Filter and Process For Safely Ionizing A Field Of An Electrically Enhanced Filter U.S. Pat. No. 5,403,383, describes an ionizing electrically enhanced filter that has sufficiently high performance to have become the only successfully commercialized Electrically Enhanced Filter (i.e., EEF). It has found uses in cleanrooms and in other critical applications, and also in residential and commercial building applications requiring clean indoor air. Recently, Consumer Reports (February 2003) rated a device based on the teachings of this patent as being the highest performance residential air cleaner.
The main advantages of electrically enhanced filtration technology are high filtration efficiency with low-pressure drop, higher filter dust holding capacity of life, and low resistance to air flow, the safety of these devices constructed with electrically enhanced technology and the ability of these devices to function without problems for the duration of the life of the product; these filters also have some bactericidal properties.
In contrast, non-EEF type conventional mechanical filters exhibit a higher pressure drop. Embodiments constructed according to the principles of U.S. Pat. No. 5,403,383 are limited as a practical matter, to relatively shallow filter media with peak-to-peak depths of about six inches.
Recent advances in filter construction have resulted in the availability of very low-pressure drop mechanical filters. For example, a class of filters known as mini-pleated V-pack filters have lower pressure drop than older deep filters such as aluminum separator type folded media and other conventional filters. A typical V-pack filter is about twelve inches deep and has a filter efficiency of 99.99% with a particle size of 0.3 micrometers, and has a pressure drop of about one inch water column at a filter face flow velocity of 600 feet per minute. Another grade of such a V-pack filter has a filtration efficiency of 95% at 0.3 micrometers particle size, and has a pressure drop of about one-half of an inch water column (i.e., 0.5″ WC) at a filter face air flow velocity of 600 feet per minute. I have found that if such a 95% filter could be enhanced in a safe electrical manner to provide approximately 99.97 to 99.99% filtration efficiency at 0.3 micrometer particle size (commonly referred to as HEPA filtration efficiency), then an ultra low pressure drop HEPA filter could be achieved with significant savings in operational costs than are available with conventional HEPA filters. Similarly lower grade, deep V-pack or other forms of deep filter material could be safely electrically enhanced to produce higher efficiency filters having significantly lower pressure drops. The operating cost savings would be in terms of fan power required and the longevity of the filter, improvements that result in savings in terms of energy, downtime, labor and material costs related to filter replacement and maintenance. The consequential benefits in industrial applications (cf. Jaisinghani, “Energy Efficient Cleanroom Design”, 2000) could be as high as 60% savings in energy consumption related to air moving.
Cheney and Spurgin in their Electrostatically Enhanced HEPA Filter, U.S. Pat. No. 4,781,736 describe an EEF that can be used with deeply folded filter media that has corrugated aluminum separators positioned within the folds. Cheney '736 is limited to using such separators as electrodes within folded dielectric filter media in paper form. The essential objective of Cheney '736 is an attempt to provide electrostatic augmented filtration that allows retrofitting or direct use of existing filters (referring to aluminum corrugated separator deep filters). Cheney '736 requires corrugated separators used as electrodes placed within folded media; if the electrodes in Cheney '736 were flat, those electrodes could not function as separators.
I have noticed that filters such as those taught by Cheney '736 rely upon sets of spacers to separate the filter media in an effort to reduce pressure drop and resistance to the air flow. I have found that this undesirably reduces the surface area of filter media available to remove particles from the air flow, principally due to the fact that these spacers have a minimum depth to the corrugations which restricts the number of pleats that can be used within an available volume. By contrast, mini pleat technology that uses glue beads or ribbons to separate the pleats enables approximately twice as much filter media when used in a V-pack configuration. Another problem that I have discovered, related to the use of aluminum separators, is that under fluctuating flow or start up flow conditions these sharp corrugated separators can cut the delicate fiber glass media used in such filters, causing damage and leakage within the filter media.
Embodiments of the Cheney and Spurgin disclosed in their U.S. Pat. No. 4,781,736 reference are also restricted to the use of an ionizer that uses parallel plates because the flow is parallel to the air flow direction. I have noticed that there are problems with parallel ionizer plates attributable to dust particles of opposing charge that tend to accumulate on the ionizer plates because the dust particles have to travel only across the direction of the air flow in order to accumulate on the plates. As highly resistive dust builds up an accumulation on the plates, an opposing field can be created, thereby canceling the applied field strength that ionizes the air. I have observed that this phenomenon can sometimes generate undesired back corona discharge.
Cheney '736 also sought a significant reduction in the capacitance of the device in comparison to the teachings of Masuda found in U.S. Pat. Nos. 4,357,150 and 4,509,958, in order to minimize the energy available for arcing. Although it is unclear whether this method may reduce the energy available for arcing as compared to Masuda '150 and '958, it reduces neither arcing and the consequent damage to the media nor the potential for fire, because pin holes can be created on the delicate glass media even with low energy arcing. Embodiments of Masuda are highly prone to arcing.
I have also found that a device constructed in accordance with Cheney '763 lacks a uniform electrical field, exhibits a low collector field strength, demonstrates a high potential for sparking, tends to have excessive leakage current, and requires construction of its frame from non-conductive materials, as is explained in the following discussion.
In order to prevent sparking towards the frame material, the frame material in the practice of Cheney '736 must be a non-conductive material, typically wood, because the aluminum spacers of the upstream corrugated electrodes will probably contact the frame material at some location. Contemporary manufacturing methods have switched to the use of aluminum or metal channel frames that do not shed particles, provide better seals to the media and are not flammable. The use of organic materials for the frames as suggested by Cheney '736 is rather dirty, and thus undesirable for clean room applications.
It should be noted that Cheney '736 does not describe any values for electrode gaps or ranges of voltages used in any of the configurations illustrated, nor does Cheney '736 provide any results showing the efficacy of the embodiments disclosed. These practical difficulties and limitations upon performance are the main reason why a device such as taught by Cheney '736 has never been successfully commercialized. Additionally, aluminum separator folded filter type filter elements have become unpopular because this type of filter element tends to tear due to the sharp edges of the aluminum separators within the folded medium.